1. Field of the Invention
This invention relates to electric power inverters of the type in which the switching devices which generate the AC waveform are controlled by digital pulse signals. In particular, the invention is directed to the generation of the pulse signals using a microprocessor and to the modification of the pulse signals to remove the DC content from the AC waveform.
2. Prior Art
Inverters are electrical devices incorporating an arrangement of electronic switches which are operated in a prearranged pattern to generate a single or multiphase AC output voltage of a desired frequency from a DC voltage input. Conventionally, analog signals generated by reactive elements have been used to control the electronic switches. Recently, attention has been directed to using digital signals to operate the electronic switches. In such an arrangement, the electronic switches are turned full on or off to generate a series of pulses in the output. The switching times are selected to form output pulses which, when filtered, produce a sinusoidal output waveform. In one such prior art inverter, the digital signals for generating a three-phase output waveform are formed by storing a series of binary numbers in a ROM (read only memory). A clock is used to sequentially output the stored numbers in a continuously repeating pattern. Selected bits in the outputted number control the conduction state of each of the electronic switches. Thus, by appropriate selection of the coded numbers, the desired inverter output pulse pattern is generated.
The electronic switches in an inverter are paired so that conduction by one switch in the pair generates the positive half cycle of the output current waveform and conduction by the other switch in the pair generates the negative half cycle. In order to generate the ideal waveform, each switch in the pair must be on for one half of the total duration of each cycle of the output waveform and off for one half the time, no matter how many pulses are generated per cycle. However, due to variations in the characteristics of the switches, such as switching time and saturation voltage, it is inevitable that one switch in a pair will generate more volt-seconds per half cycle than the other. Although this difference in volt-seconds generated is usually very small, over a period of time it results in the introduction of a DC component in the output waveform.
In the prior art digitally controlled inverter discussed above, any DC content detected in the output waveform is removed by adjusting the pulse width of two pulses in each phase exhibiting DC content. The pulse width is modulated by diverting the selected pulses through a circuit which lengthens or shortens the selected pulse depending upon the polarity of the detected DC content and the polarity of the pulse. For instance, if the DC content is positive and the pulse is positive, the pulse is shortened. In this arrangement, only the trailing edge of the pulse can be modified and the propagation time of the diverted pulse has to be selected such that it is reinserted in the pulse train at the proper instant.